Hydrogen gas has properties of odor free and low ignition energy which results in large explosion power. Therefore, in order to exactly monitor leakage of hydrogen gas in a pipeline for city gas or others and detect the leakage of hydrogen gas in an industrial hydrogen gas line, usability of a system in which a lot of small-sized hydrogen gas sensors are arranged to monitor leakage locations has been reviewed. For example, in a hydrogen concentration which is several percentages of a concentration of explosion limit, a hydrogen gas sensor which can detect hydrogen gas at a high response speed (for example, in a range of 1 to 3 seconds) is required. An idea of such a monitoring system is described in, for example, Fuel Cell, VOL, 4, No. 4, 2005, p. 60-63 (Non-Patent Document 1). In order to achieve such a monitoring system, for the hydrogen gas sensor, it is desired to respond to a hydrogen concentration which is 10% of 100 ppm at a high speed and to be low power consumption, small sized, and low cost. Also in monitoring the hydrogen leakage in fuel cell vehicle or hydrogen vehicle, a hydrogen gas sensor with low power consumption is useful.
Incidentally, there is a lithium cell (lithium battery) as one of batteries which can take the most current capacity recently. The lithium cell has a risk of ignition. However, in a case of up to two of them, it is legally established that it is not required to take a countermeasure based on a specific mounting structure for firing safety measures. When a hydrogen gas sensor is operated by using two lithium batteries with high current capacity (for example, a voltage of 3 V and a current capacity of 2.6 Ah) of the lithium cells, if power consumption is 1 mW, continuous operation of the hydrogen gas sensor for about 650 days is possible. That is, as long as the operation for one year, power consumption up to 1.78 mW is allowed. Further, when a hydrogen gas sensor is operated by using one small-sized button cell for a mobile device, even if it is a lithium cell with relatively large capacity, a voltage is 3 V but a current capacity is such small as 0.61 Ah. Therefore, in order to operate the hydrogen gas sensor for one year or longer, an idea for further reduction in the power consumption is required. The power consumption of the hydrogen gas sensor is defined mainly by a sensor unit and an interface circuit. The power consumption of the interface circuit can be reduced by configuring the interface circuit to be an IC.
As a hydrogen gas sensor whose mass production is easy and which has a property of low power operation with using Si semiconductor, a hydrogen gas sensor of a Si-MISFET (Metal Insulator Semiconductor Field Effect Transistor) type has been proposed. However, even in the Si-MISFET-type hydrogen gas sensor, if the high-speed response is required, it is required to heat the Si-MISFET-type hydrogen gas sensor at 100 to 200° C.
The present inventor has achieved the power consumption of about 100 mW in the Si-MISFET-type hydrogen gas sensor on which a sensor chip having a size of 2 mm square is mounted (see in, for example, Fuel Cell, Vol. 8, No. 3, (2009), p. 88 to 96 (Non-Patent Document 2)). However, as described above, in the cell operation, further reduction in the power consumption is required.
In the Si-MISFET-type hydrogen gas sensor in the above-described Non-Patent Document 2, a sensor MISFET, a reference MISFET, and a PN-junction-type diode for measuring the temperature are surrounded on a surface of the sensor chip by a heater wiring made of metal, and the whole sensor chip having a flat dimension of 2 mm×2 mm and a thickness of 0.4 mm is heated at 100 to 150° C. In this case, a lot of lead wires are used.
Further, as a substrate for the sensor chip, a Si substrate made of monocrystalline Si is generally used. However, the Si substrate has a thermal conductivity of 148 W/(m·° C.). Therefore, as a technique for the power reduction, an MEMS (Micro Electro Mechanical Systems) structure is known, in which a Si substrate is bored and a heater is formed in the bored region. For example, U.S. Patent Application Laid-Open Publication No. 2004/0075140 (Patent Document 1) discloses a technique in which, a Si substrate is bored, an insulator such as a Si3N4 film or a SiO2 film having low thermal conductivity is thinly formed in the bored portion, a flammable gas sensor with using a thin film such as a SnO2 film in the portion and a heater formed of a resistor body are formed and heated at about 400° C. However, in this example, the MISFET is not formed in the bored portion.
Further, for example, U.S. Pat. No. 6,111,280 (Patent Document 2) discloses a flammable gas sensor in which, an SOI (Silicon On Isolator) substrate is used, a Si-MISFET formed in a bored region in a Si substrate is used as a heater, and a sensitive membrane (for example, an SnO2 film) is arranged on the Si-MISFET to be used as a gas sensor film. Further, it discloses a hydrogen gas sensor in which, a sensitive film made of metal oxide to which a material having catalytic function is doped is formed on a gate insulating film of the Si-MISFET formed in the bored region in the Si substrate, and a Pt electrode is formed on the sensitive film. Further, it also discloses to form a heater made of a resistor body is formed on a rear surface of the SiO2 film in the bored region in the Si substrate and to heat the Si-MISFET. However, in such a structure of a hydrogen gas sensor, a manufacturing step such as performing a plurality of lithography processes to the rear surface of the SOI substrate having concave-convex surfaces is used after a lithography process to a front surface of the SOI substrate.
Meanwhile, if the low power consumption cannot be sufficiently achieved by continuous current flow, an intermittent operation method is effective. The intermittent operation method can be used, in which, heating time is represented as “τ1” and heating stop time is represented as “τ2”, and the power consumption for heating can be effectively reduced by a duty ratio “(τ1/(τ1+τ2))”. This intermittent operation method is effective when a time constant determined by the product of a thermal capacity of a targeted region with a thermal resistance connected to the targeted region is sufficiently shorter than the heating time τ1 and the heating stop time τ2. In this method, if the heating stop time τ2 is long, the power consumption can be reduced with no limit in principle. However, performance of the detection of gas leakage is reduced.
Because of limitation of the response speed of the conventional hydrogen gas sensor, it is determined in current regulations that the response speed of the hydrogen gas sensor is 30 seconds or shorter. Therefore, if a hydrogen gas sensor with a high response speed (for example, about one second) is provided, the explosion by the leakage of hydrogen gas can be prevented without damaging safety even in such an intermittent operation in which heating is performed for 2 seconds by the heater, and then, heating is stopped for 28 seconds. Therefore, a lower limit of the duty ratio is about 1/14. That is, when two lithium cells which are not regulated legally are used for, for example, a cordless monitoring system or a sensor node of a wireless monitoring system in the above-described Non-Patent Document 1, the lower limit of the power consumption is expressed as 25 mW×(1/14)≅1.78 mW. Therefore, it is considered that an upper limit of the power consumption in the heater heating for which the operation for one year can be ensured is about 25 mW.
In the Si-MISFET-type hydrogen gas sensor, on which the sensor chip having a size of 2 mm square (in which a thickness of the Si substrate is 0.4 mm) is mounted, achieved in the above-described Non-Patent Document 2, the thermal capacity of the sensor chip whose size is 2 mm square is such large as about 270 μW-second/° C. Therefore, when a chip temperature is raised from an environmental temperature (for example, −35° C.) to about 150° C., the arrival time t0 to the temperature is obtained by the quotient of the product of the thermal capacity C with a temperature difference ΔT and the heater power Pow supplied, and it takes about 1 second in 100 mW and takes about 100 seconds in 1 mW. In such a hydrogen gas sensor, when the heater is continuously operated, the arrival time t0 does not become a problem since it does not rapidly change the sensor temperature. However, when the intermittent operation of the heater is achieved, it becomes a large interruption. That is, in the existing publicly-known techniques, there is no hydrogen gas sensor equipped with a heater which can be used by a cell for a long period.
Further, an installation environmental temperature of the hydrogen gas sensor used is in a wide temperature range from a low temperature of −50° C. to a high temperature of 70° C. in some cases, and the installation is required in some cases in a place where the environmental temperature in the installation changes every day or every year. For example, in a catalytic-combustion-type hydrogen gas sensor described in, for example, Kiyoshi Fukui, Surface Technology, Vol. 57, No. 4, (2006), p. 244 to 249 (Non-Patent Document 3) or others, the problem of temperature change is solved by using a temperature compensation device, that is, a balance circuit (a Wheatstone bridge circuit or others) having a structure equivalent to that of the hydrogen gas sensor.
Further, in a Pt-gate Si-MISFET-type hydrogen gas sensor with using Pt for a gate, there are problems such that reliability for a long term cannot be guaranteed because an adhesive property between Pt and oxide such as SiO2 is poor and that a Pt film is partially peeled off, which results in contamination of a manufacturing apparatus by the peeled-off Pt film in a working process for manufacturing the Si-MISFET. Therefore, when Pt is used, the adhesive property is maintained by inserting barrier metal such as Ti, Mo, or W between Pt and SiO2, so that the contamination due to the peeling off is avoided. However, when the hydrogen gas sensor is operated, if such a barrier metal layer exists, the hydrogen gas is blocked or stored by the barrier metal layer, and therefore, there is a problem that the sensor cannot be used as the hydrogen gas sensor because the hydrogen gas sensor does not react with the hydrogen gas at all or the hydrogen response speed is significantly low (see in, for example, S. Y. Choi, et. al, IEEE Electron Device Letters, EDL-5, 14-15 (1984) (Non-Patent Document 4)).
In a Pd-gate Si-MISFET-type hydrogen gas sensor obtained by forming Pd catalytic on a gate insulating film (SiO2 film) by a sputtering method, such an effect that thermal treatment at 100° C. with 1% air-diluted hydrogen gas by significantly shorten the response time for the 1% air-diluted hydrogen gas from 50 hours to 55 seconds has been found (see in, for example, Y. Morita, et. al, Sensors and Actuators, B33, 96-99 (1996) (Non-Patent Document 5)). Note that, when irradiation of the 1% air-diluted hydrogen gas is stopped, hydrogen response intensity slightly remains so that the time required for the stop of the hydrogen response is about 655 seconds. The above-described Non-Patent Document 5 reports that, while threshold voltages Vth of an n-channel-type MISFET and a p-channel-type MISFET are 1.3 V and −0.6 V before hydrogen annealing, respectively, both of the threshold voltages obtained after the thermal treatment at 100° C. with the 1%-air diluted hydrogen gas are changed to 0.2 V.